When a fresh a deuce is so large that it breaches the surface of the toilet bowl water, exposing the tip to air.

The toilet bowl is designed to use water to contain the terrifying smell of fecal matter. Naturally, when a load is large enough to reach periscope depth it stinks up the bathroom something terrible, requiring a courtesy flush.

"Whoa dude, did you just dump all of Bombay in this bowl? My fuckin nose is going to fall off"
"Sorry after all that built up sushi that dump reached periscope depth"